Field
The invention generally relates to electronics, and in particular, to output stage of an amplifier.
Description of the Related Art
An output stage is commonly configured to deliver the output of a system, such as an amplifier, a measurement or control device or another functional block in an electrical system. Some considerations in designing an output stage include introduction of distortion, ability to drive the expected load from the following stage in the signal chain, relatively low power consumption, and ability to deliver an output signal as close to the supply rails as possible.
Some of these requirements can be more relaxed or stringent depending upon the particulars of a system. One of the challenging requirements in a measurement system such as an RF detector, is that the output voltage should be scaled with well-defined scaling parameters, so that the output voltage is as true of an indication of the signal being measured as possible. Reducing the number of scaling parameters can be helpful in achieving such a goal. In case of voltage input and output signals, a simple scaling parameter such as a resistor-ratio can be important in the translation from the input to the output. One of the requirements that a system can place on an output stage includes allowing the output voltage to swing to a supply rail, such as zero-volts, when no signal is present at the input. For a relatively small input signal, such as a few millivolts, the output should accurately produce a small output voltage as well.
Unlike system with multiple power supplies, in a single supply system, it can be challenging to have the output working range be as close to zero-volts as possible. While the output voltage can be shifted up by adding an output pedestal voltage, that introduces an additional scaling parameter which defines the pedestal voltage. The accuracy of this new scaling parameter may affect the accuracy of the overall output, especially near the bottom end of the range, where signal to be measured might itself be just a few millivolts.
Therefore an output stage that accurately produces an output voltage close to ground can be desirable for such a system.